Outlet from a boiler PRV

[Bob Minchin]

I'm planning the plumbing side of a new boiler install (Gassafe man will
be doing the gas side)
It is a standard W-B boiler 24Ri
I have an external expansion vessel fitted with a PRV
I'm not sure on the requirements for the discharge pipe from the PRV.
Ideally I'd like to go straight through the wall in copper and down to a
rainwater gully terminating the pipe just above the gully grating.
I feel that this will give an air break to stop back siphoning and a
visible means of noticing a leak. Any jet of hot water will be directed
downwards safely into a trap full of cold water.

PRV has a 1/2bsp outlet, so I assume 15mm copper will be adequate?


The WB installation manual shows a pipe through a wall with a simple
downward facing bend as soon as it exits the wall - seems more dangerous
than my method

Will my method be OK?

Searching through building regs, G3 talks about discharge from unvented
hot water tanks and the need for a tundish etc but I've yet to find
chapter and verse on boiler PRV discharge and I really don't want a
tundish indoors.



TIA

Bob

[Onetap]

On Monday, May 20, 2013 11:00:04 AM UTC+1, Bob Minchin wrote:

> Will my method be OK?

Yes.

You need a tundish on an unvented water heater because it's permanently connected to the mains water supply.

A boiler should lock out on low pressure after discharging the volume of water in the expansion vessel, so no continuous waste of water.

[Gazz]

"Bob Minchin" <bob.minc...@YOURHATntlworld.com> wrote in message
news:kncs0b$6h9$1...@dont-email.me...

Think the usual method is to take a bit of copper pipe through the wall,
then put 2 x 90 degree bends on the end, so any discharge is directed
against the wall,

There may be regs on where this can be situated, mine is about a foot to the
left of the front door and about 2 foot higher than the door top, mind, the
cowb.... plumbers fitted the condensate drain to go out the wall above the
lintel above the front door, then run along the top of the lintel to the
end, and that's it,

Never had it freeze up, but the acidic water is turning the brickwork white
where it runs down the wall, and the rain ledge part of the lintel fell off
about 6 months after the new boiler was fitted.... it was already loose due
to water getting in and rusting the rebar, thus expanding and cracking the
concrete, so i can only guess condensate water filled the cracks up and
froze, and crack, off came the rain ledge bit,

btw, rented house, and the landlord isnt too fussed, otherwise i'd have
changed the arangement myself.

[Tim+]

Cowboy installation! I rather suspect that the landlord must have been the
installer or else he *would* be fussed. Still ought to be but he is
clearly not that smart.

Tim

[Bob Minchin]

Thanks Onetap

[John Rumm]

On 20/05/2013 11:00, Bob Minchin wrote:
> I'm planning the plumbing side of a new boiler install (Gassafe man will
> be doing the gas side)
> It is a standard W-B boiler 24Ri
> I have an external expansion vessel fitted with a PRV
> I'm not sure on the requirements for the discharge pipe from the PRV.
> Ideally I'd like to go straight through the wall in copper and down to a
> rainwater gully terminating the pipe just above the gully grating.
> I feel that this will give an air break to stop back siphoning and a
> visible means of noticing a leak. Any jet of hot water will be directed
> downwards safely into a trap full of cold water.

Yup that sounds fine.

> PRV has a 1/2bsp outlet, so I assume 15mm copper will be adequate?

Yup.

> The WB installation manual shows a pipe through a wall with a simple
> downward facing bend as soon as it exits the wall - seems more dangerous
> than my method

The usual idea is to minimise risk to someone close to it. So into a
gully as you suggest is perhaps the best solution, however that is
frequently not available. So a short drop and turning it toward the wall
is a common alternative.

> Will my method be OK?
>
> Searching through building regs, G3 talks about discharge from unvented
> hot water tanks and the need for a tundish etc but I've yet to find
> chapter and verse on boiler PRV discharge and I really don't want a
> tundish indoors.

No need for a tundish on a boiler pressure relief valve - its connected
to the primary circuit which is not connected to the mains water supply,
so there is no back contamination risk (as there would be with an
unvented cylinder). The systems usually have other ways of telling you
it has operated - typically by not firing next time due to lack of pressure!

--
Cheers,

John.

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[Bob Minchin]

I wonder if I can use a simpler option still.
I plan the bring the condensate through the wall to coincide with a
rainwater downpipe and drill a hole in the back of the pipe and feed the
condensate pipe in through a grommet. I feel this is a good method as
there will be minimal exposed pipework to freeze eg about 75mm.
The rainwater has its own soakaway which I understand is acceptable for
the condensate.

I could use a similar method into the same pipe for the PRV. On the
assumption that there will only be a short spurt of return temperature
water until the pressure drops below 3bar, this should be perfectly safe
as well as being discrete.

Any thoughts?

tia

Bob

[John Rumm]

I would be wary about taking a narrow bore condensate pipe outside, even
only for a few inches. It would be ok if you transition to something
bigger first though...

> I could use a similar method into the same pipe for the PRV. On the
> assumption that there will only be a short spurt of return temperature
> water until the pressure drops below 3bar, this should be perfectly safe
> as well as being discrete.

It will be safe, but there is some argument that its advantageous to be
able to see the end of the pipe for cases where the PRV opens and does
not reseal correctly.

[Bob Minchin]

OK Thanks John - points noted.

For the condensate pipe, I could fettle an offcut of the 68mm downpipe
to fit tightly between the wall and the curved surface of the pipe and
fill it with insulation. Should prevent freezing and look even neater.
Shame that they dont make T joints for rainwater pipes.

I think I'm fixated on a discrete solution as next door had cowboys in
and their condensate pipe has a long run externally going around window
frames etc in 22mm plastic with interior grade grey foam insulation.
Looks firkin awful!

[John Rumm]

Yup, that would probably do it.

> I think I'm fixated on a discrete solution as next door had cowboys in
> and their condensate pipe has a long run externally going around window
> frames etc in 22mm plastic with interior grade grey foam insulation.
> Looks firkin awful!

Yup, I can imagine...

I just took mine into a tee I put into the main 40mm pipe after the
basin, and WM traps etc. (the boiler also has a trap that empties
syphonically every so often rather than allowing the condensate to
"dribble" out - so that also helps make freezing less likely)

[Bob Minchin]

Thanks John,
My nearest waste pipe is about 5 metres away and the run would be
through a load bearing pier and all behind kitchen units so that ain't
going to happen!
I think my boiler (WB 24Ri) has some sort of siphon built in the trap.
It is being delivered in the next few days so I can have a look in detail.
I might add a few turns of insulated wire around the pipe under the
insulation so that if I do have a problem I can connect a very low
voltage-lots of amps supply to warm the pipe up a little.

Bob

[Gazz]

>> I could use a similar method into the same pipe for the PRV. On the
>> assumption that there will only be a short spurt of return temperature
>> water until the pressure drops below 3bar, this should be perfectly safe
>> as well as being discrete.
>
> It will be safe, but there is some argument that its advantageous to be
> able to see the end of the pipe for cases where the PRV opens and does not
> reseal correctly.

most PRV's have a red knob on the end, you turn the knob and it lifts the
valve off it's seat, but it also rotates the valve too, if you keep turning
it, at half way it will snap closed again,

when i had a motorhome with a calorifier in it, the PRV came with
instructions to turn the red knob a couple of turns every now and then to
ensure any lime scale deposits are broken up and flushed away, but that PRV
was operated by pressure every trip, as the calorifier would be full, and as
the water in it was heated to about 90 degrees from the engine, it had to
expand, despite the 5 litre expansion vessel, which had to be set for the
more normal 75 degree temps the eberspacher took the calorifier upto,

So i guess more chances of a bit of limescale getting caught between the
valve and it's seat... but i only recall it weeping 2 or 3 times in the 7
years i had it.

Hence if the valve operated normally, and then didn't re-seal after you
fixed the fault that caused it to blow, give the red knob a few turns to see
if it'll clear what ever is stopping it sealing.

[John Rumm]

On 22/05/2013 22:01, Gazz wrote:
>
>>> I could use a similar method into the same pipe for the PRV. On the
>>> assumption that there will only be a short spurt of return temperature
>>> water until the pressure drops below 3bar, this should be perfectly safe
>>> as well as being discrete.
>>
>> It will be safe, but there is some argument that its advantageous to
>> be able to see the end of the pipe for cases where the PRV opens and
>> does not reseal correctly.
>
> most PRV's have a red knob on the end, you turn the knob and it lifts
> the valve off it's seat, but it also rotates the valve too, if you keep
> turning it, at half way it will snap closed again,

Indeed many combi valves are of a similar type.

> when i had a motorhome with a calorifier in it, the PRV came with
> instructions to turn the red knob a couple of turns every now and then
> to ensure any lime scale deposits are broken up and flushed away, but
> that PRV was operated by pressure every trip, as the calorifier would be
> full, and as the water in it was heated to about 90 degrees from the
> engine, it had to expand, despite the 5 litre expansion vessel, which
> had to be set for the more normal 75 degree temps the eberspacher took
> the calorifier upto,
>
> So i guess more chances of a bit of limescale getting caught between the
> valve and it's seat... but i only recall it weeping 2 or 3 times in the
> 7 years i had it.

On combis it recommended that the valve not be manually operated. Since
its on the primary side of the system its not in contact with fresh
water, and so scale ought not be a problem. However there is the risk
that debris in the primary circuit can get on the valve seat and stop it
from resealing correctly, causing persistent pressure loss problems.

> Hence if the valve operated normally, and then didn't re-seal after you
> fixed the fault that caused it to blow, give the red knob a few turns to
> see if it'll clear what ever is stopping it sealing.

Fresh water heating systems have different issues. Hence why part of the
service procedure on an unvented cylinder is to manually operate the
pressure relief valves to test them and ensure they are not scaled.